Question: Which of these numbers is prime? ${13,\ 22,\ 35,\ 81,\ 87}$
A prime number has exactly two factors: 1 and itself. The factors of 13 are 1 and 13. The factors of 22 are 1, 2, 11, and 22. The factors of 35 are 1, 5, 7, and 35. The factors of 81 are 1, 3, 9, 27, and 81. The factors of 87 are 1, 3, 29, and 87. Thus, 13 is a prime number.